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Does Science Support Field Sobriety Tests?


The validity of field sobriety tests is a controversial subject. Police officers certainly believe that they are an effective way to determine an individual’s sobriety. But is there any science to back up their belief?

The Facts

Research on field sobriety tests goes back to the 1970s. It all started when the National Highway Traffic Safety Administration decided to look into the accuracy of field sobriety tests. They contracted the Southern California Research Institute to compare the accuracy of the various field tests. And the results are the reason field tests are what they are today.

The Research Institute came up with three field tests that had a high rate of accuracy. First, there’s the “Walk and Turn Test.” Second, there’s the “One Leg Stand” test. Finally, there’s the “Horizontal Gaze” test. Although there have been more recent studies that examined the accuracy of the sobriety tests, they all confirm the original findings.

So how accurate are the field sobriety tests? A study in 2006 estimated that police officers correctly determined a driver’s impairment using a field test 91% of the time. However, that doesn’t take into account the fact that police officers can make mistakes. They’re not perfect, and it is possible for them to mishandle one of the sobriety tests. Chances are, an officer will arrest you if he suspects that you are drunk because whether you fail or pass is at his discretion.

Other Findings

Contradicting the findings of the Southern California Research Institute, a Clemson University professor did his own research project. He asked 14 officers to determine whether a driver was over or under the legal limit. After showing officers videos of 21 drivers taking six different types of field sobriety tests, the professor determined that their accuracy was at only 46%.

More evidence contradicting the high accuracy rate of field sobriety tests comes from the history of the research. When the research was first performed in the 70s, about 50% of the drivers would have been wrongly arrested (because they were under the legal BAC limit). But then, in 1981, they were able to repeat the research and achieve only 30% of wrongful arrests. Since then, the rate of error has dropped to below 10%. Is this because they’ve perfected the field tests, or because of other reasons?

There are two likely reasons for the big increase in accuracy. Most of the subjects in the study had very high BAC levels, making it more obvious that they were drunk. Further skewing the data, officers were allowed to use Breathalyzer results in their assessment.

Walk and Turn Test

Some people call this the “Walk the Line” test because, well, that’s exactly what you have to do. A police officer asks you to walk on a straight line with your eyes down. You have to walk heel to toe and count out nine steps. Then, you need to pivot around and do it once more.

This test has an accuracy of 79%. It tests how well you can follow instructions- a skill dulled by alcohol. While administering the test, officers look for poor balance, stepping off the line, an inability to walk heel to toe, turning too soon or too late, and taking the wrong number of steps. They look for poor balance and any signs that you can’t follow instructions.

One Leg Stand Test

The One Leg Stand Test is 83% accurate and checks your balance and concentration. An officer asks you to stand on one leg with the other leg lifted about six inches off the ground. Then, he asks you to count out loud and watches you for 30 seconds. If you sway, hop, or put your foot down, he counts those actions against you.

Horizontal Gaze Test

The least reliable of the three tests is the gaze test. During the test, an officer asks a driver to follow his finger (or an object) with his eyes. The officer then moves his finger side to side. As he does this, he looks for involuntary eye movements. These eye movements may indicate drunkenness.

This test has an 88% accuracy rate. But that rate doesn’t account for one thing- the fact that there are other conditions that can cause an involuntary eye movement. This includes medical conditions, age, and medication. Not everyone who fails this test is drunk. Some people just can’t control their eye movements.

Why are field sobriety tests important?

Rhode Island has an implied consent law. This means that an officer can get a breath sample from you without your consent. However, these samples are usually taken after an arrest. If a police officer stops you because he suspects you are driving under the influence, he needs to perform a field sobriety test before he can arrest you. A failed test gives the officer probable cause to arrest you. But an inaccurate field test could mean that an officer arrests you without probable cause.

If an officer uses a field sobriety test that is not one of the three mentioned in this article, the evidence against you may be thrown out in court. If you’ve been accused of drinking and driving, you need an experienced lawyer to handle the case.

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